Process for Preparing an Alpha-Lipoic Acid/Cyclodextrin Complex and Product Prepared

ABSTRACT

The invention relates to a process for the preparation of a cyclodextrin/alpha-lipoic acid complex, wherein in a first step, an alpha-lipoic acid and a cyclodextrin are dissolved in an aqueous alkaline solution having a pH above pH 7, and in a second step an acid is added to lower the pH of the solution to a pH below pH 7.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is division of U.S. application Ser. No. 11/302,810filed Dec. 14, 2005. The disclosure of which is incorporated in itsentirety by reference herein.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a process for preparing an alpha-lipoicacid/cyclodextrin complex and product prepared.

2. Background Art

Cyclodextrins are cyclic oligosaccharides composed of 6, 7 or 8α(1-4)-linked anhydroglucose units. The α-, β- or γ-cyclodextrins, whichare prepared for example by enzymatic conversion of starch, differ inthe diameter of their hydrophobic cavity and are generally suitable forinclusion of numerous lipophilic substances.

Alpha-lipoic acid (Thioctic Acid) is a naturally occurring moleculeproduced by both plants and animals. It serves as a cofactor for someenzymes and as an excellent antioxidant/free radical scavenger. Since ithas been known that alpha-Lipoic Acid can improve insulin sensitivity incases of Type II diabetes, alpha-Lipoic Acid has been employed fortreating diabetes as well as alcoholic liver disease and otherneuropathies.

Alpha-lipoic acid is a lipid-soluble substance. It is a yellowish powderand is unstable in light. Its solubility in water is very poor. Aformulation with cyclodextrins, especially with alpha cyclodextrin, isstable to temperature and light and increases the dispersibility inwater and the bioavailability. The complexes also reduce the unpleasantodour of alpha-lipoic acid.

The complexation of lipoic acid with a-cyclodextrin and β-cyclodextrinusing a solution method is described in the Journal of InclusionPhenomena and Molecular Recognition in Chemistry, (1995), vol. 23, pp.119-96. The processes described therein for preparing a cyclodextrincomplex with alpha-lipoic acid show various disadvantages in thelaboratory and production scale. On use of the solution method on theproduction scale, the amount of water needed to solubilize thecyclodextrin is enormous. For economic reasons, therefore, these methodscannot be used commercially to prepare a cyclodextrin/alpha-lipoic acidcomplex. According to this literature the thermal stability ofalpha-lipoic acid can be improved significantly by its inclusion in betacyclodextrin. Calculation of the stability constants for thealpha-lipoic acid/cyclodextrin complex according to theBenesi-Hildebrand procedure are described also on page 121 of thispaper. The calculated stability constants for the cyclodextrin complexwith alpha cyclodextrin were 3.34 and for the beta cyclodextrin complex3.95. According to these findings the beta cyclodextrin complex is morestable than the alpha cyclodextrin alpha-lipoic acid complex.

In Canadian patent application 2,135,535 a process is describedcharacterized in that thioctic acid is suspended in water, cyclodextrinsare added at elevated temperature such as 50° C., stirred for severalhours, cooled down and the inclusion compound is isolated by filtrationand vacuum dried. Working in suspension to produce cyclodextrinscomplexes requires elevated temperatures and long complexation times forefficient results. Elevated temperatures can have a negative impact onthe stability of the alpha-lipoic acid.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a process whichallows preparation of a cyclodextrin/alpha-lipoic acid complex in arapid and uncomplicated manner, even at room temperature, without thedisadvantages of the prior art. These and other objects are achieved bya two step process which comprises dissolving an alpha-lipoic acid and acyclodextrin in an aqueous alkaline solution having a pH above 7,followed by step adding an acid to lower the pH of the solution to a pHbelow pH 7. By this process a mixture of solids in water comprising thecyclodextrin/alpha-lipoic acid complex is obtained without hightemperatures and long mixing times. The costs of the production processare reduced significantly.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

The cyclodextrins used are α-, β-, or γ-cyclodextrin, preferably β- orα-cyclodextrin, most preferably α-cyclodextrin. The alpha-lipoic acidused is either racemic or enantiomerically pure. The solvent ispreferably a water/alcohol mixture, most preferably water.

The process preferably starts from an aqueous alpha-lipoic acid solutionabove pH 7 to which the cyclodextrin is added. However, it is equallypossible to prepare an aqueous cyclodextrin solution above pH 7 firstand to add lipoic acid thereto. The pH of the aqueous solution duringthe first step is preferably between pH 10 and pH 14, more preferablybetween pH 13 and pH 14, or at pH 14. The pH above pH 7 is preferablyadjusted by using a suitable base, most preferably a base selected fromamong ammonium hydroxide, sodium hydroxide, and potassium hydroxide. Thesolubilization of the alpha-lipoic acid and the cyclodextrin in thefirst step is effected using a base and standard mixing equipment. Itshould be noted that when amorphous silica is employed as a flowingagent in either reactant, the solution will still be slightly cloudy.

The solids content of the aqueous solution in the first step ispreferably less than 50% (w/w). The solids content can be calculated bydividing the sum of the dry weights of the cyclodextrin, alpha-lipoicacid, and base, by the overall weight of the mixture. The aqueousmixture in this first step preferably contains a solids content between10 and 50% by weight, more preferably between 20 and 40%, and mostpreferably between 25 and 35%.

The molar ratio cyclodextrin to alpha-lipoic acid is preferably between2.0 and 1.0, more preferably between 1.5 to 1.0, yet more preferablybetween 1.2 and 1.0, and most preferably between 1.15 and 1.0.

The process of the invention preferably takes place in a temperaturerange of 10-40° C., more preferably at 15-30° C., and most preferably atabout 25° C. The complexation ordinarily takes place under atmosphericpressure. The reaction (complexation) time of the first step of theprocess is preferably below 3 h, more preferably below 2 h, and mostpreferably below 1 h.

In the second step of the process according to the present invention theformed alpha-lipoic acid/cyclodextrin complex is precipitated byaddition of an acid. An inorganic or organic acid may be used,preferably an inorganic or organic acid selected from among hydrochloricacid, sulphuric acid, phosphoric acid, acetic acid, citric acid, andascorbic acid. Especially preferred are hydrochloric acid, sulphuricacid, and phosphoric acid.

The second turns the lipoic acid back to its acid form, precipitates thecomplex and brings the pH of the final aqueous mixture below pH 7. Thefinal pH of the aqueous mixture in this second step is thus preferablybetween pH 6 and pH 3, more preferably between pH 5 and pH 3.

The complex can be used directly in the form of the reaction mixture,however, it may also be isolated e.g. by filtration, centrifugation,belt drying, or spray-drying methods, and processed to give a stablepowder. The complex is preferably dried by oven drying, spray drying,filtration or belt drying, most preferably by spray-drying in a spraydryer having an inlet temperature higher than 160° C. and an outlettemperature below 100° C., yielding a fine yellow-white powder. Theinlet spray drying temperature preferably is higher than 170° C. with anoutlet temperature below 90° C., most preferably higher than 180° C.with an outlet temperature below 80° C.

The alpha-lipoic acid/cyclodextrin complexes produced according to thisinvention show a higher degree of complexation and provide bettertemperature stability in powder form than complexes produced by themethods. Therefore the invention also relates to an alpha-lipoicacid/cyclodextrin complex produced according to the process of thepresent invention. An alpha-lipoic acid complex produced according tothe present invention contains less than 10% of non-complexed material,preferably less than 8%, and most preferably less than 5% measured usingDifferential Scanning Calorimetry (see e.g. example 7).

By contrast, an alpha-lipoic acid/cyclodextrin complex producedaccording to the Canadian patent application 2,135,535 shows about 28%of non-complexed alpha-lipoic acid (see comparison example and example7). The method of the present invention produces an alpha-lipoicacid/cyclodextrin complex with significantly lower percent ofnon-complexed alpha-lipoic acid, even up to 100% complexation rate (seeexample 7). The superiority of complexes produced according to themethod described in this patents leads to significantly highertemperature stability (see example 8, 9, and 10).

According to published heating experiments, at 100° C. the half life oflipoic acid at 100° C. is 5.2 h. For the alpha-lipoic acid/betacyclodextrin complex it is 73 h (see Journal of Inclusion Phenomena andMolecular Recognition in Chemistry, (1995), vol. 23, page 122 and FIG.4). From these measurements it can be calculated the beta CD complex isabout 14 times more stable at 100° C. (73h/5.2h) than the alpha-lipoicacid itself. The alpha cyclodextrin/alpha-lipoic acid complex accordingto the present invention shows a half life of more than 213 days and isthus at least 70 times more stable than the lipoic acid itself (seeexample 10).

The complex according to the present invention may be used inpharmaceutical formulations, dietary supplements, and nutraceutical, orfunctional food formulations.

The following examples are intended to illustrate the invention ingreater detail, but should not be construed as limiting the scope of theinvention in any way. The α-, β-, and γ-cyclodextrins can be purchasedunder the name Cavamax™ cyclodextrins from Wacker-Chemie, Munich,Germany. Alpha-lipoic acid can be purchased from Degussa, Freising,Germany. The lipoic acid may also contain a flowing agent such asamorphous silica.

Comparison Example C1:

(According to example 7 of the Canadian Patent Application 2,135,535)

172.8 g alpha cyclodextrin containing approx. 10% water were dissolvedin 700 g water at 50° C. 3.3 g Sodium ascorbate was added and dissolvedin the alpha cyclodextrin solution. 33.01 g sieved thioctic acid wasadded to the alpha cyclodextrin solution and stirred 3 hours at 50° C.The suspension was then filtered and the filtrate was cooled (50° C. 12h). The precipitate hereby formed was separated by filtration and driedin a vacuum oven at 30°. This product contained 15.8% alpha-lipoic acidby weight according to HPLC analysis (see example 6) and was used forcomparison purposes in examples 7 and 10.

Example 1 Preparation of a Cyclodextrin Alpha-Lipoic Acid AlphaCyclodextrin Complex

117.2 g of a 45% potassium hydroxide solution (0.940 mol) was added to5.0 kg water and mixed at room temperature until clear. 194.0 galpha-lipoic acid (99%, 0.931 moles) was added to the potassiumhydroxide solution and mixed until clear. 1,046.0 g alpha cyclodextrin(9.25% water content, 0.977 moles) was added and stirred 3 hours untilslightly cloudy to clear (in case of the amorphous silica as flowingagent the solution will be slightly cloudy). 0.940 mol of 12 Nhydrochloric acid was slowly added until the pH reached pH 3 toprecipitate the complex. The final aqueous mixture is spray driedimmediately at 180° C.

Example 2 Preparation of an Alpha-Lipoic Acid/Alpha Cyclodextrin Complex

186.7 g of a 45% potassium hydroxide solution (1.498 moles) was added to3910 g water and mixed at room temperature until clear. 300 galpha-lipoic acid (99%, 1.439 moles) was added to the potassiumhydroxide solution and mixed until clear clear (in case of the presenceof amorphous silica as flowing agent the solution will be slightlycloudy). 1,572 g alpha cyclodextrin (at 9.25% water 1.468 moles) wasadded and stirred for 3 hours at room temperature until no furtherclarification was seen. 1.498 moles of 37% hydrochloric acid was addedto precipitate the complex. With a minimal molar excess of hydrochloricacid in comparison to the potassium hydroxide used, the final pH of thesolution was at pH 3. The final aqueous mixture was spray driedimmediately at 180° C.

Example 3 Preparation of an Alpha-Lipoic Acid/Alpha Cyclodextrin Complex

1.44 moles of potassium hydroxide were added to 5.5 kg of water andmixed at room temperature until clear. The pH of this solution was at pH14. 300 g alpha-lipoic acid (98%, 1.425 moles) was added to thepotassium hydroxide solution and mixed until clear (in case of thepresence of amorphous silica as flowing agent the solution will beslightly cloudy). 1.553 kg of dry alpha cyclodextrin (1.598 moles) wasadded and stirred 50 minutes at room temperature. 1.44 moles of 37%hydrochloric acid was added precipitating the complex. This mixture wasstirred 30 minutes at room temperature. The final pH was at about pH 3.This aqueous mixture was spray dried immediately at about 180° C. inletand about 80° C. outlet temperature.

Example 4 Preparation of an Alpha-Lipoic Acid/Alpha Cyclodextrin Complex

23.9 g of a 45% potassium hydroxide solution (0.192 moles) was added to767 g water and mixed at room temperature until clear. 40.0 galpha-lipoic acid (98%, 0.190 moles) was added to the potassiumhydroxide solution and mixed at room temperature until clear (in case ofthe presence of amorphous silica as flowing agent the solution will beslightly cloudy). 221.7 g alpha cyclodextrin (9.2% water content, 0.207moles) was added and stirred for 60 minutes at room temperature. 0.192moles of 37% hydrochloric acid was added to precipitate the complex.With a minimal molar excess of hydrochloric acid in comparison to thepotassium hydroxide used the final pH of the solution was about pH 3.The final aqueous mixture is spray dried immediately at 180° C.

Example 5 Preparation of an Alpha-Lipoic Acid/Alpha Cyclodextrin Complex

29.95 kg of potassium hydroxide pellets (87.9%, 469.3 moles) were addedto 1920.0 kg of water and mixed at room temperature until clear. The pHof this solution was at pH 14. 97.8 kg alpha-lipoic acid (98%, 464.5moles) was added to the potassium hydroxide solution and mixed untilclear (in case of the presence of amorphous silica as flowing agent thesolution will be slightly cloudy). 555.1 kg alpha cyclodextrin (8.9%water content, 520.3 moles) was added and stirred 60 minutes at roomtemperature. 469.3 moles of 37.7% hydrochloric acid was addedprecipitating the complex. This mixture was stirred 55 minutes at roomtemperature. The final pH was at about pH 3. The final aqueous mixturewas spray dried immediately at about 180° C. inlet and about 80° C.outlet temperature.

Example 6 High-Performance Liquid Chromatography Analysis

The alpha-lipoic acid/cyclodextrin complexes were analyzed for contentusing High-Performance Liquid Chromatography (HPLC). The column used isa 25 mm C 18, and the conditions are as follows: UV detector atwavelength 215 nm, flowrate 0.8 ml/min. Mobile phase is 585 ml ethanol,90 ml Acetonitrile, 455 ml 0.05 mol potassium dihydrogen phosphate(KH₂PO₄), adjusted to pH 3.0-3.1 with phosphoric acid (H₃PO₄). Theinjection volume was 20 microliters (μl) and the column temperature 30°C. Under these conditions the retention time for alpha-lipoic acid isapproximately 6.5 minutes.

Procedure:

150 mg of cyclodextrin complex was sonicated for 60 seconds in 50 mlacetonitrile. 40 ml water was added and sonicated for another 30seconds. The solution was transferred to a 100 ml volumetric flask andfilled to the mark with water. This solution was then tested by HPLC andthe alpha-lipoic acid content content calculated using a multipointcalibration curve prepared from a known standard.

Results:

Alpha-lipoic Acid Content HPLC Samples (w/w) [%] Comparison Example C115.8 Example 1 13.4 Example 2 13.6 Example 3 12.5 Example 4 13.5 Example5 12.7

Example 7 Differential Scanning Calorimetry Analysis for UncomplexedAlpha-Lipoic Acid

Cyclodextrin complexes (3-6 mg sample size) were analyzed usingDifferential Scanning Calorimeter (DSC). Samples were scanned for analpha-lipoic acid (ALA) melting peak around 60° C. This peak, ifpresent, was compared to a peak at this temperature of pure alpha-lipoicacid to calculate percentage of uncomplexed alpha-lipoic acid in thecomplex. Taking the sample size and the alpha-lipoic acid content of thesample into account from the area under the curve (“AUC”) the meltingenergy in J/g for uncomplexed alpha-lipoic acid can be calculated. Thepeak around 60° C. decreases with increasing degree of complexation. Acyclodextrin complex containing no uncomplexed alpha-lipoic acid showsno melting peak around 60° C.

Results:

Com- Content Sample AUC at Peak at Non plexed ALA Weight 60° C. 60° C.Complexed ALA [%] [mg] [mJ] [J/g] ALA [%] [%] Pure 98.0 3.057 377.40 126100 0.0 ALA Comp. 15.8 4.890 27.526 36 28.3 11.3 Ex. Ex. 3 12.5 2.890 00 0 12.5 Ex. 4 13.5 4.040 0 0 0 13.5 Ex. 5 12.7 5.662 2.673 4 3.2 12.3

Even with a lower alpha-lipoic acid content in the complex, the amountof complexed alpha-lipoic acid is significatly higher in productproduced according to the present application (12.3% to 13.5% versus11.3% in the comparison example).

Example 8 Temperature Stability at 25° C.

Alpha lipic acid/cyclodextrin complexes were stored in a closedpolypropylene container at 25° C. Samples were taken from time to timeand the alpha-lipoic acid content was measured via HPLC according toexample 6. The initial alpha-lipoic acid content was set to 100%.

Results:

Days of Content ALA [%] Content ALA [%] storage Pure ALA Example 5 0 100100 5 100 14 100 39 100 45 97 67 100 76 97 107 100 97 135 97

The pure alpha-lipoic acid is stable at room temperature. Thealpha-lipoic acid in the cyclodextrin complex according to thisinvention is also stable at room temperature after some small initiallosses, which correspond to the uncomplexed alpha-lipoic acid in thecomplex (the complex in example 5 has 3% uncomplexed alpha-lipoic acidaccording to DSC measurements in example 7).

Example 9 Temperature Stability at 40° C.

Alpha-lipoic acid/cyclodextrin complexes were stored in a closedpolypropylene container in a drying oven at 40° C. Samples were takenfrom time to time and the alpha-lipoic acid content was measured viaHPLC according to example 6. The initial alpha-lipoic acid content wasset to 100%.

Results:

Days of Content ALA [%] Content ALA [%] storage Pure ALA Example 5 0100    100 5 97 8 99    14 96 28 94 39 48*) 45 93 59 87 76 86 81 41*)107 86 135 86 *)difficult to obtain a homogeneous sample, becauseproduct polymerizes

The alpha-lipoic acid in the cyclodextrin complex according to thisinvention shows a significantly higher stability than the purealpha-lipoic acid and stays stable at 40° C. after some initial losses.

Example 10 Temperature Stability at 100° C.

Alpha-lipoic acid/cyclodextrin complexes were stored in closed glasscontainers in a drying oven at 100° C. Samples were taken from time totime and the alpha-lipoic acid content was measured via HPLC accordingto example 6. The initial alpha-lipoic acid content was set to 100%.

Results:

Days of Content ALA [%] Content ALA [%] Content ALA [%] storage Pure ALAComparison Example Example 3 0 100 100 100 3 38 85 6 96 9 93 11 13 13 9114 77 24 72 28 4 36 0 53 80 55 35 84 77 112 72 125 10 143 66 173 60 21354

The half life of pure alpha-lipoic acid under the described storagecondition at 100° C. is less than three days. The half life ofalpha-lipoic acid complex (example 3) is more than 213 days. This meansthe complex produced according to this invention under these conditionsis more than 70 times more stable than the pure alpha-lipoic acid itself(213d/3d). Even after 213 days of storage at 100° C. still 54% of theoriginal alpha-lipoic acid content is not degraded, whereas only 10% canbe detected in product produced according the Canadian patent 2,135,535just after 125 days.

While embodiments of the invention have been illustrated and described,it is not intended that these embodiments illustrate and describe allpossible forms of the invention. Rather, the words used in thespecification are words of description rather than limitation, and it isunderstood that various changes may be made without departing from thespirit and scope of the invention.

1. An alpha-lipoic acid/cyclodextrin composition comprising: analpha-lipoic acid/cyclodextrin complex including less than 10 dry weightpercent of non-complexed alpha-lipoic acid, based on DifferentialScanning Calorimetry.
 2. The alpha-lipoic acid/cyclodextrin compositionof claim 1, wherein the alpha-lipoic acid/cyclodextrin complex has ahalf life of more than 213 days at 100 degrees Celsius.
 3. Thealpha-lipoic acid/cyclodextrin composition of claim 1, wherein the alphalipoic acid/cyclodextrin complex includes less than 8 dry weight percentof non-complexed alpha-lipoic acid.
 4. The alpha-lipoicacid/cyclodextrin composition of claim 1, wherein the alpha-lipoicacid/cyclodextrin complex includes less than 5 dry weight percent ofnon-complexed alpha-lipoic acid.
 5. The alpha-lipoic acid/cyclodextrincomposition of claim 1, wherein a molar ratio of cyclodextrin toalpha-lipoic acid in the alpha-lipoic acid/cyclodextrin complex isbetween 2.0 and 1.0.
 6. The alpha-lipoic acid/cyclodextrin compositionof claim 1, wherein a molar ratio of cyclodextrin to alpha-lipoic acidin the alpha-lipoic acid/cyclodextrin complex is between 1.5 and 1.0. 7.The alpha-lipoic acid/cyclodextrin composition of claim 1, wherein amolar ratio of cyclodextrin to alpha-lipoic acid in the alpha-lipoicacid/cyclodextrin complex is between 1.2 and 1.0.
 8. The alpha-lipoicacid/cyclodextrin composition of claim 1, wherein a molar ratio ofcyclodextrin to alpha-lipoic acid in the alpha-lipoic acid/cyclodextrincomplex is between 1.15 and 1.0.
 9. An alpha-lipoic acid/cyclodextrincomposition, comprising: a liquid mixture of an alpha-lipoicacid/cyclodextrin complex and a liquid, the liquid mixture having a pHless than pH
 7. 10. The alpha-lipoic acid/cyclodextrin composition ofclaim 9, wherein alpha-lipoic acid of the alpha-lipoic acid/cyclodextrincomplex is in an acidic form.
 11. The alpha-lipoic acid/cyclodextrincomposition of claim 9, wherein the liquid includes water.
 12. Thealpha-lipoic acid/cyclodextrin composition of claim 9, wherein theliquid includes a mixture of water and an organic solvent.
 13. Thealpha-lipoic acid/cyclodextrin composition of claim 9, wherein thealpha-lipoic acid/cyclodextrin composition has a solids content of lessthan 50 percent by weight.
 14. The alpha-lipoic acid/cyclodextrincomposition of claim 9, wherein the liquid mixture has a pH between pH 6and pH
 3. 15. The alpha-lipoic acid/cyclodextrin composition of claim 9,wherein the alpha-lipoic acid/cyclodextrin complex includes less than 10dry weight percent of non-complexed alpha lipoic acid, based onDifferential Scanning Calorimetry.
 16. The alpha-lipoicacid/cyclodextrin composition of claim 9, wherein a molar ratio ofcyclodextrin to alpha-lipoic acid of the alpha-lipoic acid/cyclodextrincomplex is between 2.0 and 1.0.
 17. The alpha-lipoic acid/cyclodextrincomposition of claim 9, wherein the liquid includes an organic solvent.18. An alpha-lipoic acid/cyclodextrin complex having a half life of morethan 213 days at 100 degrees Celsius, produced by the processcomprising: in a first step, dissolving alpha-lipoic acid and acyclodextrin in an aqueous alkaline solution having a pH above pH 7, andin a second step adding an acid to lower the pH of the solution to a pHbelow pH
 7. 19. The alpha-lipoic acid/cyclodextrin complex of claim 18,wherein the aqueous solution during the first step is between pH 10 andpH 14, and wherein the molar ratio of cyclodextrin to alpha-lipoic acidis between 2.0 and 1.0, and wherein the final pH of the aqueous solutionin the second step is between pH 6 and pH
 3. 20. The alpha-lipoicacid/cyclodextrin complex of claim 18, which includes less than 10 dryweight percent of non-complexed alpha-lipoic acid.